1. Field of the Invention
The present invention relates to operational amplifiers, and more specifically to operational amplifiers having a phase compensating circuit.
2. Description of Related Art
At present, operational amplifiers are widely used in various fields, particularly as one important element in analog circuits. Ordinarily, the operational amplifier fundamentally comprises two amplifier stages, i.e., an initial amplifier stage having an inverted input and a non-inverted input, and an output amplifier stage having an input connected to an output of the initial stage. This output stage ordinarily includes a capacitive feedback circuit between the output and the input of the output stage. However, in the case that the feedback path is established by only a simple capacitive electrical part, the gain of the operational amplifier does not sufficiently drop at a higher frequency region. This will often give rise to a problem of null or zero points, which lead to a cause for oscillation of the operational amplifier.
In order to avoid such an adverse effect of the zero points, various types of zero point compensating circuits have heretofore been proposed. However, almost the conventional zero point compensating circuits will make worse the supply voltage rejection ratio (abbreviated "SVRR" hereinafter) of the operational amplifier.
Among such conventional amplifiers having the zero point compensating circuits, one amplifier has succeeded not only in enabling the zero-point compensation but also in improving the SVRR, by forming the feedback circuit by series-connected capacitor and active load. In this amplifier, however, the active load is required to have a large transconductance, which will result in increase of power comsuption or increase in the element dimensions necessary for the active load, leading to increase in the integrated circuit chip size.